Television waveform monitors are used to provide a display in the time domain of the waveform of a baseband composite video signal, and to enable measurements to be made on those signals. A measurement that is typically made is the horizontal sync duration or width. Sync width is measured by observing the distance on the display between the -20 IRE points of the leading and trailing edges of the horizontal sync pulse. Knowing the horizontal magnification of the monitor, the sync width can be inferred.
A typical waveform monitor has a display screen that is about 4 inches (about 10 centimeters) wide, and at the lowest horizontal magnification setting two lines of the video signal can be displayed. For the sake of clarity, the line that is seen to the left will be referred to hereinafter as the first line and the line that is seen to the right, and has its sync pulse at about the middle of the displayed waveform, will be referred to as the second line. It will, however, be appreciated that there is no necessary numerical relationship between the positioning of the displays of the two lines and the numbering of the fields of the video signal, except to the extent that such a relationship may be imposed by the design of a particular waveform monitor.
The nominal sync width is about 4.7 .mu.s, or about 7.4 percent of the duration of a single line of a video signal. This implies that at the lowest horizontal magnification setting the sync pulse occupies about 3.7 percent of the width of the display of a waveform monitor. Clearly, with a display screen that is about 10 cm wide, it is not possible to make accurate measurements on a feature that occupies less than 4 percent of the screen width simply by naked eye observation of graticules engraved on the display screen. Therefore, in order to make accurate measurements it is necessary to expand the displayed waveform in the horizontal direction by increasing the horizontal magnification so that the sync pulse occupies considerably more than 3.7 percent of the width of the display. In a conventional waveform monitor, however, the sensitivity of the instrument to horizontal position of the waveform increases as the horizontal magnification is increased. This is because the conventional waveform monitor often employs a single turn potentiometer to move the waveform horizontally on the display screen, and rotation of the potentiometer from its center position (in which the sync pulse of the second line is at about the center of the screen) through a predetermined angle (e. g. about 180 degrees in the case of a single turn potentiometer) causes the waveform to be displaced so that one end of the waveform (depending on the direction of rotation) is shifted right across the display screen, regardless of the horizontal magnification setting of the monitor. Consequently, when the horizontal magnification is increased ten-fold so that the sync pulse occupies about one-third of the width of the screen, the horizontal position control becomes ten-times more sensitive, and it is very difficult to position the sync pulse accurately relative to the graticule markings on the display screen with a single turn potentiometer.